1. Field of the Invention
This invention relates broadly to transmission systems. More particularly, this invention relates to transmission systems that transmit power from a microturbine engine.
2. State of the Art
Small low-cost unmanned air vehicles (UAVs) have been developed and deployed to carry out a variety of military roles such as reconnaissance and attack missions. Currently, intermittent combustion piston engines of 100 HP (or less) power all of the low speed UAV aircraft. Most of these engines drive propellers without the need for a gearbox. However, these engines burn gasoline, which is highly flammable and thus undesirable for field service operations. Piston engines also have undesirable vibration characteristics and are difficult to start in cold weather operations.
Locust USA, Inc of Miami, Fla., assignee of the present invention has developed a microturbine engine and gearbox for use in UAVs. The microturbine engine, when used in conjunction with heavy jet fuel (such as JP-8 fuel), provides a highly advantageous propulsion system for UAVs due to its lighter weight, use of less flammable fuels, higher reliability and reduced vibrations. Microturbine engines operate at very high rotational speeds, typically in the range between 50,000 RPM and 250,000 RPM with an output power between 200 HP and 5 HP. UAVs operate at much slower propeller rotational speeds, typically on the order of 2000 RPM to 7000 RPM. These constraints dictate that the gearbox provide a reduction ratio on order of 25:1 to 36:1 over the RPM range and horsepower range of the microturbine engine.
In the current Locust designs, two different types of spline drive couplers as well as two different types of axial retention mechanisms can be used to couple the output of the microturbine engine to the input of the gear box. The two spline drive coupler types include an outside diameter piloted spline coupler (sometimes referred to as a “flat root major diameter fit spline coupler”) and a polygon spline coupler. The two types of axial retention mechanism include a snap ring and set screws.
FIGS. 1A and 1B illustrate an outside diameter piloted spline coupler wherein the output spline 2 of the microturbine engine includes projections 6 that project radially inward from the inner diameter surface to engage the piloted section 5 of the input shaft 3 of the gear box. The outside diameter piloted spline coupler is well suited for the high RPM speeds produced by the microturbine engine. Moreover, the axial travel of the input shaft 3 relative to the output spline 2 that is permitted by the outside diameter piloted spline coupler provides for simple and efficient assembly and disassembly of the coupling between the engine and the gear box.
In the configuration of FIG. 1A, a snap ring 8 is disposed within an annular groove in the inner diameter surface of the output spline 2 and projects radially inward to engage the input shaft 3 in a manner that limits the axial travel of the input shaft 3 relative to the output spline 2. The snap ring 8 is problematic because it is very difficult to assemble due to the fact that the snap ring is “trapped” in a small annular groove on the outside diameter surface of the input shaft 3.
In the configuration of FIG. 1B, set screws 9 are screwed through holes that pass from the outside diameter surface to the inside diameter surface of the output spline 2 and project radially inward to engage the input shaft 3 in a manner that limits the axial travel of the input shaft relative to the output spline 2. The set screws 9 are easier to install than the snap ring but can present problems with unbalance at very high engine speeds and are prone to wear.
In small size applications, it may be difficult to manufacture the teeth (projections) of the inside diameter piloted spline coupler. As an alternative in such small size applications, a polygonal spline coupler as shown in FIG. 1C can be used. This exemplary polygonal spline coupler employs a three-lobe polygonal design. This design can be manufactured by very accurate grinding of the polygonal system to produce accurate fits.
Thus, there remains a need in the art to provide a mechanism for coupling a microturbine engine to a gear box in a manner that is easy to assemble and disassemble while also providing for improved reliability, and to incorporate such a mechanism in a small lightweight propulsion system for use in UAVs.